Matrix metalloproteinase (MMP) family consists of Zn2+-dependent endopeptidases. The enzymes degrade structural proteins of basement membranes and extracellular matrix facilitating tissue remodeling and cell mobility thus phenomena accompanying physiological and inflammatory processes as well as cancer diseases. One of the subgroups among MMPs is a subfamily of gelatinases consisting of gelatinase A (MMP-2) and gelatinase B (MMP-9). The two enzymes are characterized by the most complicated structure among MMPs and similar spectrum of substrates, however, they show different expression constitutive in case of MMP-2 and (usually) inducible of MMP-9. Moreover, one of distinguished features of MMP-9 is the capacity to regulate cytokine and chemokine activity. Bioengineering of transgenic mice deficient in gelatinase B (MMP-9-/-) revealed also physiological functions of MMP-9, for example in reproduction, functioning of nervous system, bone development, remodelling of blood vessels, wound healing and some processes occurring in thymus. Moreover, studies on MMP-9-/-mice allowed for detailed studies of the role of the enzyme in inflammation. It was shown that apart from its role in tissue infiltration by inflammatory leukocytes (mostly neutrophils), during late stages of inflammation MMP-9 also participates in apoptosis of the cells. Furthermore, it was reported that in certain circumstances MMP-9 might limit neutrophil infiltration by degradation of various chemokines. In addition development of compensatory mechanisms was reported to operate in MMP-9-/- mice. Thus overall MMP-9 is one of the key enzymes in the development and the course of inflammation.